In prior art, a light bulb assembly of a light string, when it is used outdoors, must be waterproof to prevent from infiltrating water or moisture. The known structure of the light bulb assembly including a bulb holder with a bulb and a housing is not effective for obtaining a perfect sealed connection. The water or moisture would be infiltrated in the housing and contacts with two conductive wires at the same time that becomes a short circuit and damages the light bulb assembly. It is a main drawback to be overcome.